To use water reasonably so as not to waste water is a duty for every person. A traditional outlet device usually just has a simple valve and a water flow depends on the diameter of a pipe, because the diameter of water pipe usually is quite larger and thus a lot of water will spray out when the valve opened. Accordingly, larger amount of the water is wasted. So various water saving devices have been invented and manufactured by the persons in the field, such as a water saving gasket for tap disclosed in CN2846943 which is a protrusion gasket having a flow-restrictor hole in the middle portion. The effective area of the flow-restrictor hole is 5˜80 mm2, and the preferred area is 10˜50 mm2. However, this gasket just reduce the outlet diameter so as to reduce the flow, it can neither adjust the flow nor make the flow steady.